POP3 and IMAP stop respond

I have Postfix + Courier-POP3/Courier-IMAP configured on Debian etch.
From times to times POP3 and IMAP stop working.
This is the case now that when I try to use POP3 I get the following alert: “Connection to mail… timed out” or “Your POP3 server does not respond…”
When using IMAP I get the following: “Login to server failed …”

The system was working normally two days ago and no changes where made in configurations.
Could someone help getting out from this?
Thanks.